1. Field of the Invention
This invention pertains to ignition systems and more particularly to spark igniters for burners and burner pilots.
2. Description of the Related Art
A gas burner pilot is a device used to create a stable pilot flame by combustion of a low flow rate (relative to the main burner) gaseous fuel-air mixture. The pilot flame is used to light a larger main burner, or a difficult to light fuel. Gas pilot designs normally include an ignition system. One common type of ignition systems used in gas burner pilots, as well as other systems such as flare systems, is a high-energy ignition (HEI).
An HEI system typically utilizes a capacitive discharge exciter to pass large current pulses to a spark rod. The large current pulses are often greater than 1 kA. The spark igniter (also known as spark plug, spark rod or igniter probe) for an HEI system is generally constructed using a center electrode surrounded by an insulator and an outer conducting shell over the insulator such that, at the axially-facing ignition end of the spark rod, an air gap is formed between the center electrode and the outer conduction shell, i.e., a gap between the center electrode and the outer electrode shell or conducting shell. At this air gap, also called a spark gap, a high-energy spark can pass between the center electrode and outer conducting shell. Often a semiconductor material is applied to the insulating material at this gap to facilitate sparking. HEI systems have the ability to maintain powerful high energy sparks in adverse conditions such as cold temperatures, heavy fuels (heavy gases or oils), contamination of the igniter plug with coking or other debris and moisture presence due to steam purging or rain.
Past HEI spark igniter designs produced sparks on an axial-facing surface (referred to herein after as “axial-directed spark igniter”). One variable that affects spark energy is the size of the air gap on the axial-facing surface of the igniter. As the air gap increases, the amount of energy released during the spark event also increases. Air gaps generally range in size from 1 mm to 2 mm.
The center electrode, the electrode shell and semiconductor material erode away as sparking occurs over the course of an igniter's life. An igniter generally reaches the end of its life when either the semiconductor has worn away or when the air gap has become too large due to electrode erosion. Thus, while there is a desire to have relatively large air gaps because fuel ignition is more likely with higher energy release, problems are encountered with increasing the air gap size. Increased air gap size means either a shorter igniter life due to less material used in the center electrode and/or electrode shell or a larger higher-cost igniter due to an increased outer shell diameter and, hence, increased material. It would be desirable to have an igniter allowing for an increased gap size without significantly increasing the size or amount of material used and without adversely affecting igniter life.
In addition to the above considerations, the igniter life can be shortened by the exposure of the semiconductor material to flame radiation. In some burner pilot configurations, the flame may root in a position in which the igniter's semiconductor material is exposed to flame radiation. Flame radiation damages the semiconductor material, which generally reduces the life of an igniter. Accordingly, it would be desirable to avoid this problem in a burner pilot design.